In common communication networks, in particular in mobile communication networks two main techniques for a data transmission are used: circuit switched data transmission and packet switched data transmission.
In the circuit switched domain a network establishes a service connection for data transmission by allocating a (radio) channel to a terminal, e.g. a mobile station, when a network host or service provider intends to transmit data via the network. Then data is transmitted via the network after the connection has been established. The radio channel is occupied by the terminal during the entire duration of the connection, even though in many cases only a small amount of data has to be transmitted.
In the packet switched domain the network transmits a data packet only when required, i.e. when data transmission has to be carried out. Thus, several terminals can use the same channel at a time. If a terminal generates a data packet, the network routes that packet via a first unattached channel to a recipient. Thus, as data transmission frequently consists of data bursts, the channels can be used in an efficient manner. This type of packet switched data transmission is used in the Internet and e.g. GPRS (General Packet Radio Service) systems and UMTS (Universal Mobile Telecommunications System) systems.
Data packets are routed in packet switched networks via mostly unpredictable routes depending on the actual data traffic and depending on the configuration of the corresponding routers. Thus, packets may encounter different delays inside the network caused e.g. by variation in occupancy levels of queues inside the routers. Furthermore, as a result, the network capacity, in particular within certain routes, has to be divided between different glows (or connections, or customers).
The division of the capacity between the different flows is controlled by the so-called SIMA (Simple Integrated Media Access) system as being described in U.S. Pat. No. 6,047,326 and U.S. Pat. No. 6,081,505.
SIMA is a way to provide QoS (Quality of Service) using differentiated services, i.e. SIMA is a complete service and implementation solution. Differentiated services is an approach to provide QoS in the internet. The basic idea is to provide relative (soft) guarantees for delivery of IP (Internet Protocol) data packets. Complex functions are left at the edges of the network. This allows simple core network routers and ensures the scalability.
According to the SIMA concept each user or application shall define only two issues before a connection is established, namely a nominal bit rate (NBR) and the selection between real-time and non-real-time service classes. NBR forms the basic relationship between charging and QoS, and it defines how the network capacity is divided among different connections during overload situations. Due to the simplicity of SIMA the network operator does not guarantee the continuous availability of NBR. Furthermore, a user or application is allowed to send data with any bit rate independently of the NBR. The strength of SIMA is its wide area of applications. SIMA does not require to build complex systems with several service classes each appropriate to only certain applications.
The Idea of SIMA is that at the network edge packets are assigned a priority for a network domain. The packets are treated in the network domain according to the priority. The priority is depending on the ratio of a measured momentary bit rate (MBR) and NBR: If the MBR exceeds the NBR the priority is dropped and if the MBR is clearly below the NBR the priority is raised. Thereby, the NBR corresponds roughly to the bandwidth allocation for a given client.
Though the SIMA model as being described in U.S. Pat. No. 6,047,326 and U.S. Pat. No. 6,081,505 has been proved to be efficient and quite flexible, it has certain limitations. One of the limitations is that most of the application using the SIMA network are assumed to be adaptive in the sense that during overload situations they can reduce the bit rate, and correspondingly, if there is free capacity they can momentarily increase their bit rate. If that assumption is valid, the network capacity is divided proportional to the NBR for each flow. Due to this property, both the service provider and the customer may consider that the position of applications with constant (unchangeable) bit rate obtains worth service that what it should be.